X-ray image tube having an oxidized vanadium barrier interposed between the scintillator and photocathode

ABSTRACT

An X-ray image tube is disclosed having a glass barrier comprising oxidized vanadium interposed between the tube scintillator and photocathode.

I United States Patent Niewold et al. Feb. 13, 1973 [54] X-RAY IMAGE TUBE HAVING AN [56] References Cited OXIDIZED VANADIUM BARRIER INTERPOSED BETWEEN THE UN'TED STATES PATENTS SCINTILLATOR AND 2,955,218 10/1960 Schmidt ..250 213 VT PHOTOCATHODE 3,278,317 11/1966 Blair et al ..250/83 CD 3,439,165 4/1969 Hopkinson et al. ..250/7l.5 [75] Inventors: Andreas Niewold, Sunnyvale; Judith 3,446,745 5/1969 Brinckmann ..250/71.5 Q. Pon, Cupertino, both of Calif. 3,462,601 8/1969 Sternglass ..250/83.3

[73] Assignee: Varian Associates, Palo Alto, Calif. Primary Examiner Archie R. Borchelt [22] Filed: Nov. 15, 1968 Att0mey-Wiiliam J. Nolan and Leon F. Herbert [2]] Appl. No.: 776,089 [57] ABSTRACT An X-ray image tube is disclosed having a glass barrier fiig l g comprising oxidized vanadium interposed between the n o I I I e e I e t b t d h t d [58] Field of Search ..250/71.5,s3.3,213 VT u aora p o oca 0 e 13 Claims, 2 Drawing Figures X-RAY IMAGE X-RAY BE A M1 2 J 2 OBJECT I PATENTED FEB 1 3197s FIG.I

FIG.2

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INVENTORS AN 5 NIEWOLD JU 0. PON

ATTORNEY X-RAY IMAGE TUBE HAVING AN OXIDIZED VANADIUM BARRIER INTERPOSED BETWEEN THE SCINTILLATOR AND PHOTOCATHODE BACKGROUND OF THE INVENTION This invention relates generally to X-ray image tubes, and particularly to the input packs of such devices.

Heretofore, the input packs of X-ray image tubes have consisted of several thin layers of distinct materials. The outer layer has frequently comprised a portion of the tube envelope and provided structural support for the input pack. It has consisted of an X-ray transparent material such as aluminum or aluminum-coated glass.

To this outer support layer has been deposited a layer of an X-ray sensitive material such as zinc sulfide to serve as a phosphor which receives and converts incident X-ray images to optical images. A layer of glass has then been deposited upon the phosphor, and an alkali antimonide in turn deposited on the glass. The alkali antimonide has served as a photocathode which receives optical images emanating from the phosphor and converts them to electron images. The layer of glass interposed between the phosphor and photocathode has served as a barrier to prevent unwanted chemical reactions from occurring between the layers of zinc sulfide and alkali antimonide.

The presence of the layer of glass has tended to detract from the sensitivity of the input pack. Means have therefore been sought to reduce or eliminate the need for the barrier. Such endeavor culminated in success recently with the discovery that photocathedic potassium antimonide could be deposited directly upon a scintillatic activated alkali metal halide such as cesium iodide without apparent adverse reactions therebetween. This discovery is detailed in copending U.S. Pat. application Ser. No. 606,513 assigned to the assignee of the present invention. Subsequent research however has revealed certain practical limitations on the use of this apparatus. Specifically it has been found that at relatively low X-ray load levels, such as 30 milliroentgens per minute and above, an interaction occurs between the abutting scintillator and photocathode giving rise to a progressive defocusing of the electron image within the input pack. More specifically, as seen from the tube viewing screen the X-ray image appears to fold and become fuzzy. The periphery of the image moves inwardly thereby compacting the image as a whole.

Accordingly, it is the general object of the present invention to provide an improved X-ray image tube.

More specifically, it is an object of the invention to provide an improved input pack for an X-ray image tube.

Another object of the invention is the provision of an improved barrier between the scintillator and photocathode of an X-ray image tube.

Yet another object of the present invention is to provide an X-ray image tube adapted to intensify and con vert an X-ray load in excess of 30 milliroentgens with an improved barrier interposed between an alkali halide scintillator and an alkali antimonide photocathode, which barrier prevents significant defocusing without a significant, attendant sacrifice in sensitivity.

SUMMARY OF THE INVENTION Briefly described, the present invention is an improved X-ray image tube having a scintillator and a photocathode. A glass barrier comprising a majority by weight of oxidized vanadium is interposed between the scintillator and photocathode.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of an X-ray system including an X-ray image tube of the present invention.

FIG. 2 is an enlarged cross-sectional view of the portion of the X-ray image tube shown in FIG. I delineated by the line 2--2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now in more detail to the drawing, there is illustrated in FIG. 1 an X-ray system including an X-ray generator which projects a beam of X-rays against an object. An X-ray image tube is positioned behind the object to receive the X-ray image thereof. Such a system is described in detail in the article titled X-Ray Image Intensification With A Large Diameter Image Intensifier Tube appearing in the American Journal of Roentgenology Radium Therapy and Nuclear Medicine, volume 85, pages 323-341 of February I961.

The X-ray image tube comprises a dielectric, evacuated envelope 1 which is approximately 17 inches long and 10 inches in diameter. The face portion of the tube includes a novel input pack 2 which is shown in greater detail in FIG. 2. The tube further comprises electron focusing electrode 3, an anode 4 and a fluorescent viewing screen 5. The input pack consists of a tube envelope member and input pack support layer 6 made of aluminum which is transparent to incident X-rays. A scintillator comprising a layer 7 of cesium iodide and sodium is deposited on the surface of support layer 6. If desired, thallium may be substituted for the sodium. A barrier comprises a layer 8 composed of a majority by weight of vanadium pentoxide and a minority by weight of phosphorus pentoxide is deposited onto the scintillator. Finally a photocathode comprising a layer 9 of cesium antimonide is deposited onto the barrier. Each deposit operation is performed by use of evaporative techniques.

In operation X-rays generated by the X-ray generator penetrate the object to be observed. The local X- ray attenuation depends on both the thickness and atomic number of the elements forming the object under observation. Thus, the intensity pattern in the X- ray beam after penetration of the object contains information concerning the structure of the object. The X- ray image then passes through support layer 6 of the tube input pack and impinges upon scintillator 7 as symbolically shown by arrow 10 in FIG. 2. Within scintillator 7 the X-ray photons are absorbed and reemitted as optical photons, typically in the blue frequency range. The optical photons pass through barrier 8 with approximately to percent transmission. The optical photons then strike photocathode 9 wherein they produce electrons e. The electrons are emitted from the photocathode in a pattern or image corresponding to the initial, incident X-ray image. The

electrons are then accelerated to a high velocity within the Xeray image tube and focused through anode 4 onto fluorescent viewing screen 5 for viewing by the eye or other suitable optical pick-up device.

The surprising success met with the use of vanadium pentoxide as a barrier between the scintillator and photocathode followed substantial experimentation with other materials such as tin oxide, tungsten trioxide, silicon oxide, silicon dioxide, platinum and aluminum. Such success would ordinarily be quite unexpected inasmuch as vanadium pentoxide is an opaque, semiconductive material rather than a transparent dielectric such as was the glass formerly used as a barrier between zinc sulfide phosphors and photocathodes.

As vanadium pentoxide is naturally yellow-red, only an extremely thin layer can be applied without the optical transmission of the barrier dropping below an acceptable level. it was found that a layer of between and 200 angstroms thick yields satisfactory results, satisfactory results meaning sufricient optical transmission without adverse interaction between the scintillator and photocathode as evidenced by electron defocusing within the input pack.

It should be understood that the just described, preferred embodiment is merely illustrative of the invention, and that the scope of the invention is intended to be defined only by the following claims.

We claim:

1. in an X-ray image tube having a scintillator and a photocathode, the improvement comprising a vitreous, at least partially conductive barrier interposed between the scintillator and photocathode, said barrier comprising a majority by weight of oxidized vanadium.

2. The apparatus of claim 1 wherein said barrier has a visible light transmission of approximately to percent.

3. The apparatus of claim 1 wherein said barrier has a thickness of between 20 and 200 angstroms.

4. The apparatus of claim 1 wherein said barrier comprises a minority by weight of phosphorus pentoxide.

S. The apparatus of claim 1 wherein the scintillator comprises an alkali halide.

6. The apparatus of claim 1 wherein the scintillatorv comprises cesium iodide.

7. The apparatus of claim 6 wherein the scintillator further comprises thallium.

8. The apparatus of claim 6 wherein the scintillator further comprises sodium.

9. The apparatus of claim 1 wherein photocathode comprises an alkali antimonide.

10. The apparatus of claim 1 wherein the photocathode comprises cesium antimonide.

11. The apparatus of claim 5 wherein the photocathode comprises cesium antimonide.

12. The apparatus of claim 1 wherein said barrier comprises vanadium pentoxide.

13. An improved X-ray image tube comprising an evacuated envelope, a scintillator to receive and convert incident X-ray images to optical images, a photocathode to receive and convert the optical images to electron images, a barrier comprising a vitreous layer of oxidized vanadium glass interposed between the scintillator and photocathode, means for accelerating and focusing the electron images and a fluorescent viewing screen for receiving and converting the focused electron image patterns to optical images corresponding to the incident X-ray images.

the 

1. In an X-ray image tube having a scintillator and a photocathode, the improvement comprising a vitreous, at least partially conductive barrier interposed between the scintillator and photocathode, said barrier comprising a majority by weight of oxidized vanadium.
 2. The apparatus of claim 1 wherein said barrier has a visible light transmission of approximately 75 to 80 percent.
 3. The apparatus of claim 1 wherein said barrier has a thickness of between 20 and 200 angstroms.
 4. The apparatus of claim 1 wherein said barrier comprises a minority by weight of phosphorus pentoxide.
 5. The apparatus of claim 1 wherein the scintillator comprises an alkali halide.
 6. The apparatus of claim 1 wherein the scintillator comprises cesium iodide.
 7. The apparatus of claim 6 wherein the scintillator further comprises thallium.
 8. The apparatus of claim 6 wherein the scintillator further comprises sodium.
 9. The apparatus of claim 1 wherein the photocathode comprises an alkali antimonide.
 10. The apparatus of claim 1 wherein the photocathode comprises cesium antimonide.
 11. The apparatus of claim 5 wherein the photocathode comprises cesium antimonide.
 12. The apparatus of claim 1 wherein said barrier comprises vanadium pentoxide. 